1. Field of the Invention
This invention relates in general to electrolytic processes and equipment for refining copper and more particularly to an improved electrolytic cathode and method of making same.
2. Description of the Prior Art
The principle of electrolysis has been utilized for decades to extract metals and other cations from an electrolytic solution. The extraction process is carried out by passing an electric current through an electrolyte solution of the metal of interest, such as copper, gold, silver or lead. The metal is extracted by electrical deposition as a result of current flow between a large number of anode and cathode plates immersed in cells of a dedicated extraction tank house. The anode is made of a material that is dissolved and therefore lost during the process, while the cathode is constructed of a metal alloy, such as titanium or copper alloys and various grades of stainless steel (316L, 2205, etc.), which are resistant to corrosive acid solutions. In the most efficient processes, each cathode consists of a thin sheet of metal having a uniform thickness (2–4 mm) disposed vertically between parallel sheets of anodic material, so that an even current density is present throughout the surface of the cathode. A solution of metal-rich electrolyte and various other chemicals, as required to maintain an optimal rate of deposition, are circulated through the extraction cells; thus, as an electric current is passed through the anodes, electrolyte and cathodes, a pure layer of electrolyte metal is electro-deposited on the cathode surface, which becomes plated by the process.
Similarly, to purify a metal in a refinery process using electro-deposition, an anode of impure metal is placed in an electrolytic solution of the same metal and subjected to an electric current passing through the anode, electrolyte and cathode of each cell. The anode goes into solution and the impurities drop to the bottom of the tank. The dissolved metal then follows the current flow and is deposited in pure form on the cathode, which typically consists of a starter sheet of stainless steel. When a certain amount of pure metal has been plated onto the starter sheet, the cathode is pulled out of the tank and stripped of the pure metal.
In both processes, the pure metal deposit is grown to a specific thickness on the cathode during a predetermined length of time and then the cathode is removed from the cell. It is important that the layer of metal deposited be recovered in uniform shaped and thicknesses and that its grade be of the highest quality so that it will adhere to the cathode blank during deposition and be easily removed by automated stripping equipment afterwards. The overall economy of the production process depends in part on the ability to mechanically strip the cathode of the metal deposits at high throughputs and speeds without utilizing manual or physical intervention. To that end, the cathode blanks must have a surface finish that is resistant to the corrosive solution of the tank house and must be strong enough to withstand their continuous handling by automated machines without pitting or marking. Any degradation of the blank's finish causes the electro-deposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
Also immensely important in the production and refining of metals by electrolytic extraction is the relationship of electrical power consumption with metal production rates. The total weight of deposited metal can be calculated theoretically by knowing the actual energy used, the concentration of metal in solution, the average residence time, the number of cells, and the surface area available for deposition in each cell. In practice, all electrical amperages and flow rates are continuously monitored throughout the deposition cycle to optimize the electrolytic process. After the cathodes have been pulled out of the cells and the deposited metals have been stripped and weighed, the electrolytic-production weight is divided by the theoretical cell production weight to determine cell efficiency. A cell efficiency of ninety-five percent or better is the goal for the best operations.
In order to achieve this level of efficiency, the voltage profile across the cathodic deposition surface must be held constant and variations avoided. Shorts due to areas of high current density caused by nodulization or by curved cathode surfaces that touch the anode must be prevented. Therefore, the details of construction of cathode blanks are very important to minimize operational problems and ensure high yields.
U.S. Pat. No. 4,186,074 issued to Perry in 1980 describes a cathode for electrolytic refining of copper that was considered to be the state of the art in the industry. It consists of a stainless steel hanger bar with the top edge of a stainless steel starter sheet in abutting relationship with the flat bottom surface of the hanger bar. Stitch welding is used to attach the starter sheet to the hanger bar so that it depends vertically from the hanger bar. The opposite ends of the hanger bar are supported on a spaced-apart pair of horizontally disposed bus bars and are in electrically conductive contact therewith for energizing the system. In order to reduce the electrical resistance resulting from the spot welds between the hanger bar and the starter sheet, the hanger bar and the upper edge of the starter sheet are uniformly clad with copper, thereby creating a low resistance boundary between the two.
The cathode structure disclosed in the Perry patent was a significant improvement over the prior art; however, some of its features caused problems from time to time. The flat bottom surface of the hanger bar tended to remain positioned in full contact with the bus bars even when the starter sheet was not perfectly perpendicular to it because of warpage or other structural defects. In such cases, the starter sheet would not hang perfectly vertical and its distance from adjacent anodes was not uniform and sometimes it would even be in shorted contact with the anodes. This caused nonuniform deposits that affected the efficiency of operation and the quality of the product. Another problem with the Perry cathode resulted from wear which caused pits and faults to develop in the copper cladding around the hanger bar. When this occurred, the steel of the hanger bar underneath the copper cladding was exposed to the highly corrosive atmosphere of the tank house and this resulted in a rapid build-up of high-resistance corrosion spots which decreased the conductivity of the entire electrode. Such corrosion eventually caused enough structural damage to require replacement of the hanger bar and reconditioning of the cathode. In addition, when the copper plating became sufficiently worn to become inefficient as a conductor at the boundary between the hanger bar and the starter sheet, the current flow became restricted to the relatively high resistance weld spots and therefore affected the efficiency of the cathode as well.
U.S. Pat. No. 5,492,609 by Assenmacher overcame some of the problems associated with the Perry cathode. The hanger bar is of solid copper and has a longitudinally extending groove formed in the bottom surface thereof into which the upper edge of the starter sheet fits tightly. A continuous seam weld is used to provide improved boundary conductivity without the need for copper plating. The hanger bar is configured with a rounded in cross-section bottom surface to allow the cathode to rotate under the influence of gravity into a vertically disposed attitude to provide uniform spacing of the cathode relative to the adjacent anodes. Although Assenmacher disclosed a significantly improved structure, some problems remained unsolved. One such problem has been the galvanic corrosion that takes place at the junction of two dissimilar metals; that is, the stainless steel starter sheet and the copper hanger bar. The welding process which joins the starter sheet to the hanger bar causes a melting of both metals, which in turn produces a commingling of the two metals and brings a relatively large amount of dissimilar metal particles into contact with each other. Therefore, galvanic corrosion is increased by the enlarged interface produced by the melting of the two metals associated with the welding process. The greater interface between copper and steel along the weld bead is also exposed to the highly corrosive atmosphere of the tank house, which causes etching into the weld bead and which in time causes a decrease in electrical conductivity and eventually structural damage to the cathode.
Therefore, a need exists for a new and improved cathode structure for electrolytic refining of copper and a method of making same, with the improved cathode overcoming some of the shortcomings of the prior art.